In early 2018, the concentration of carbon dioxide in our atmosphere exceeds 400 ppm. This high level of carbon dioxide is considered to be one of the prime contributors to global warming, which is creating rising sea levels, more destructive storms, droughts and floods.
Many methods have been proposed to capture carbon dioxide from the smokestacks of electricity generating facilities that burn carbon-based fuels (eg. coal, oil, biomass, or natural gas). Other methods have been proposed to capture carbon dioxide directly from the air. Permanently storing (a.k.a. sequestering) the captured carbon dioxide at low cost has proven to be difficult.
Various authors have proposed storing carbon dioxide under high pressure in depleted oil or gas wells and then re-capping the wells. Unfortunately, the process of drilling and extracting the oil or gas from the well creates pathways by which the carbon dioxide can escape. Also, the depleted wells are often located far from any electricity generating facility. The significant cost of transporting the carbon dioxide over long distances negatively impacts the cost effectiveness of well storage in such cases.
Other authors have proposed storing carbon dioxide under high pressure in natural caverns. As in the case of wells, discussed above, the caverns will tend to leak gas, and the cost of transporting the carbon dioxide to the caverns makes this approach not cost effective.
Other authors have proposed liquefying the carbon dioxide and storing it deep under the ocean, in the hope that the high pressures and cold temperatures deep under the ocean would hold the carbon dioxide in place for a long time. This method is both expensive and not permanent enough. The carbon dioxide must be transported and compressed to a very high pressure, which is expensive. Also, over time, it will dissolve in the sea water and diffuse to the surface.
Recently, scientists in Iceland have dissolved carbon dioxide into a large volume of water and then introduced the solution into caves of igneous rock, such as volcanic rock or basalt, where a small fraction of the dissolved carbon dioxide reacts with the rock to form stable carbonates. In another study at the Pacific Northwest Laboratory, scientists forced carbon dioxide gas under very high pressure into deposits of basalt rock located 1250 meters below the surface. In each of these two experiments, some of the carbon dioxide gas was converted to a stable solid form. Unfortunately, the amount that was converted was too small for this approach to be useful for storing large amounts of carbon dioxide.
In all of these prior art methods, there is an initial phase requiring the separation of carbon dioxide from nitrogen and other trace gases in the exhaust of carbon-based electricity generating plants, and this separation has been expensive. In many cases, it has driven the price of sequestration so high that the value of the electricity generating fuel (e.g. coal or natural gas) is greatly compromised.
There is a natural process by which certain types of rock are weathered, and by which atmospheric carbon dioxide is converted into solid carbonates. Carbon dioxide in the air dissolves in rainwater to form a weak acid, carbonic acid (H2CO3). This acid interacts with rock to create carbonates. An example of this second reaction, with the rock mineral Feldspar, one of the most abundant types of rock in the Earth's crust, is indicated below.

Hard rock and the carbon dioxide that is dissolved in water to form carbonic acid (H2O+CO2→H2CO3) are consumed. For this type of rock, the products are soluble carbonate, clay particles and SiO2 (the major component of sand). For rocks containing Ca, the carbonic acid forms Ca(HCO3)2 (calcium bicarbonate). Subsequently, water with dissolved calcium bicarbonate will react with other rocks that will raise the pH of the solution. When the pH is high enough, the calcium bicarbonate converts to CaCO3, which deposits as limestone rock. Limestone is one of the Earth's great natural permanent stores for carbon and carbon dioxide.
The natural weathering of rock is well understood, but very slow. If we stopped burning all fossil fuels now, with only natural processes, it would take 1000 years to restore the level of carbon dioxide in the atmosphere back to its levels prior to 1950.
There have also been scientific studies to understand what limits the rates at which these natural rock weathering processes proceed. Like most chemical reactions, the reaction rate for the weathering process is proportional to the surface area of the rock that is exposed, to the concentration of carbon dioxide in the water, which determines the concentration of carbonic acid in the water, and to the degree that the carbonic acid is dissociated in the water, which is determined by the acidity of the solution and the concentration of bi-carbonate (HCO3−) and carbonate (CO3− −) ions.
There is, therefore, a need for methods to sequester carbon dioxide in rock to form carbonates at rates higher than those that would occur naturally, without human intervention.